1. Field of the Invention
The present invention relates to a stable angular velocity sensor for detecting amount of movements, controlling postures, and the like, which comprises a piezoelectric vibrator, an oscillator section, and a Coriolis output detection section, that are not influenced by factors such as an ambient temperature, power supply voltage fluctuations, and an unevenness in the components, by utilizing the piezoelectric vibrator such as a crystal vibrator.
2. Related Background Art
Conventional technologies related to angular velocity sensors are disclosed in the following patent publications. Examples include Japanese Patent No. 2781161, Japanese Unexamined Patent Publication Nos. HEI 11-44540, 2002-174520, and 2003-87057.
FIG. 1 shows a principle of the angular velocity sensor of the prior art disclosed in Japanese Patent No. 2781161.
For example, if a crystal tuning fork vibrator is used as a piezoelectric vibrator 1 of the angular velocity sensor, a drive electrode 2 used in excitation and a detection electrode 3 for detecting Coriolis force are prepared on the pre-determined positions on the front face of the piezoelectric vibrator 1. An oscillator circuit 4 is connected to the drive electrode 2 for supplying an alternating current (hereinafter referred to as AC) drive voltage. Moreover, a Coriolis force detection circuit 5 is connected to the detection electrode 3.
The piezoelectric vibrator 1 having a mass m, vibrates at a pre-determined frequency in the B direction along the X-axis if the AC drive voltage is applied to the drive electrode 2 from the oscillator circuit 4. If an angular velocity ω is applied around the Y-axis, a Coriolis force F (=2mv ω) is generated in the Z-axis direction perpendicular to the X-axis (provided that v=vibration velocity of the piezoelectric vibrator 1). Since the Coriolis force F is proportional to a magnitude of the angular velocity ω, the magnitude of the angular velocity ω of the piezoelectric vibrator 1 is determined by detecting the Coriolis force F as a deflection displacement of the piezoelectric vibrator 1 at the Coriolis force detection circuit 5 and the detection electrode 3.
Angular velocity sensors are loaded on vehicles and airplanes to record their run courses and flight locus, and are used in detecting the yaw rates generated at curves. Moreover, recently, the angular velocity sensors are also loaded on robots to control their postures. The car navigation systems which are very common these days also loads them for detecting position displacement where the global positioning system (hereinafter referred to as GPS) waves do not reach. Further, they are also utilized as displacement detecting sensors for preventing the digital camera and video camera shakes.
To improve precision, the following angular velocity sensors are being proposed in Japanese Patent No. 2781161 and Japanese Unexamined Patent Publication No. HEI 11-44540.
According to Japanese Patent No. 2781161, depending on a power supply voltage applied to operate the oscillator circuit 4, a drive voltage level supplied from the oscillator circuit 4 to the drive electrode 2 is controlled by an automatic gain control (hereinafter referred to as AGC) circuit prepared at the oscillator circuit 4. Also, the drive voltage from the oscillator section 4 and a detecting signal from the Coriolis force detection circuit 5 are proportional to the power supply voltage. Because of this, if an analog/digital converter (hereinafter referred to as A/D converter) for converting an analog detecting signal output from the Coriolis force detection circuit 5 to a digital signal, which is connected to an output terminal of the Coriolis force detection circuit 5, a conversion precision of the A/D converter is prevented from changing due to the power supply voltage fluctuations applied to the oscillator circuit 4. The disclosed invention is effective in improving the conversion precision.
According to Japanese Unexamined Patent Publication No. HEI 11-44540, similar to Japanese Patent No. 2781161, an amplitude of the AC drive voltage output from the oscillator circuit 4 is made to change due to the power supply voltage fluctuations applied to the oscillator circuit 4. Moreover, a temperature sensor for detecting the ambient temperature is prepared. To the detecting signal output from the Coriolis force detection circuit 5, a temperature correction process is performed by incorporating an output of the temperature sensor into a microcomputer, by performing the A/D conversion, and further by performing a digital/analog conversion (hereinafter referred to as D/A conversion).
However, the angular velocity sensors of Japanese Patent No. 2781161 and Japanese Unexamined Patent Publication No. HEI 11-44540 do not sufficiently perform a temperature compensation at the oscillator circuits 4; therefore, the inventors of the present application have proposed the following improvements as disclosed in Japanese Unexamined Patent Publication Nos. 2002-174520 and 2003-87057.
In the case of detecting an angular velocity ω from the Coriolis force F=2mv ω generated when the angular velocity ω is applied to the piezoelectric vibrator 1, a detection precision of the angular velocity ω is improved by making the vibration velocity v of the piezoelectric vibrator 1 constant without a fluctuation caused by the oscillator circuit 4. That is, the vibration velocity v is proportional to an output current from the drive electrode 2 of the piezoelectric vibrator 1, and it is proportional to an output voltage that had converted the output current to the output voltage. Therefore, according to Japanese Unexamined Patent Publication Nos. 2002-174520 and 2003-87057, the oscillator circuit 4 is configured so that the output voltage becomes constant by compensating the temperature, and not influenced by the ambient temperature. The stability of the angular velocity sensors is improved accordingly.
However, the angular velocity sensors disclosed in Japanese Patent No. 2781161, Japanese Unexamined Patent Publication Nos. HEI 11-44540, 2002-174520, and 2003-87057 have the following technical problems.
The angular velocity sensor vibrates the piezoelectric vibrator 1 by applying the AC drive voltage output to the drive electrode 2 from the oscillator circuit 4, and detects the detecting signal of a potential corresponding to the Coriolis force F, by the detection electrode 3 and the Coriolis force detection circuit 5. The angular velocity sensor such as this, the requirement for a stability of the detecting signal is of vital importance, because of the device characteristics (such as the device configuration) of the piezoelectric vibrator itself, and as for the device voltage for vibrating the piezoelectric vibrator 1, because a signal detectable as the Coriolis force F is extremely small. Moreover, the piezoelectric vibrator 1, the oscillator circuit 4, and the Coriolis force detection circuit 5 readily receive temperature influence from the viewpoint of circuit component configuration, therefore, to the utmost, there's the need to increase the quality (that is, the stability) of the oscillator circuit 4 and the Coriolis force detection circuit 5.
However, according to the angular velocity sensors disclosed in Japanese Patent No. 2781161, Japanese Unexamined Patent Publication Nos. HEI 11-44540, 2002-174520, and 2003-87057, the temperature correction process is performed on the detecting signal output from the Coriolis force detection circuit 5, alternatively, the temperature change is controlled by the AGC circuit prepared inside the oscillator circuit 4. For example, in the case of using the piezoelectric vibrator 1 with a large value of temperature coefficient such as the crystal tuning fork vibrator, an operational range of the AGC circuit inside the oscillator circuit 4 is deviated, and an output voltage of the AGC circuit may be excessive or be insufficient. The output voltage for driving the piezoelectric vibrator 1 is not proportional to the temperature. This results in either the saturation or the termination of the oscillation. There's a danger of losing the function as the angular velocity sensor. Moreover, the technical problems such as an insufficient reduction in the startup time, an insufficient measure of the startup failure at power ON, a temperature compensation of the Coriolis force detection circuit 5, and a removal of common-mode component, had not been taken into account.